How to make the snake grow in size using C# in Unity in 3D?

I have an assignment that asked to make a retro game using Unity. My snake head can move around but cannot grow in size when collecting item. Help?

This is my script:

using System.Collections; using System.Collections.Generic; using UnityEngine;

public class control: MonoBehaviour {

 public List<Transform> BodyParts = new List<Transform>();

 public float mindistance = 0.25f;

 public int beginsize;

 public float speed = 1;
 public float rotationspeed = 50;

 public float TimeFromLastRetry;

 public GameObject bodyprefab;

 private float dis;
 private Transform curBodyPart;
 private Transform PrevBodypart;

 public bool IsAlive;

 private Rigidbody rb;
 private GameManager gm;

 // Use this for initialization
 void Start () {

     rb = GetComponent<Rigidbody> ();

     GameObject thescore = GameObject.Find ("Game Manager");
     gm = thescore.GetComponent<GameManager> ();

 }

 // Update is called once per frame
 void Update () {

     float moveHorizontal = Input.GetAxis ("Horizontal");
     float moveVertical = Input.GetAxis ("Vertical");

     Vector3 movement = new Vector3 (moveHorizontal, 0.0f, moveVertical);
     //get direction from the values

     rb.AddForce (movement * speed);
     //addForce = how strong the ball is pushed

 }

 void OnTriggerEnter(Collider other){
     if(other.gameObject.CompareTag ("Pick Up")){
         other.gameObject.SetActive(false);

         gm.score = gm.score + 1;
         gm.SetScore ();
     }
 }

 public void Move()
 {
     float curspeed = speed; //snake always move

     if (Input.GetKey(KeyCode.W))
         curspeed *=2;

     BodyParts [0].Translate (BodyParts [0].forward * curspeed * Time.smoothDeltaTime, Space.World);

     if (Input.GetAxis ("Horizontal") != 0)
         BodyParts [0].Rotate (Vector3.up * rotationspeed * Time.deltaTime * Input.GetAxis ("Horizontal"));

     for (int i = 1; i < BodyParts.Count; i++ )
     {
         curBodyPart = BodyParts [i];
         PrevBodypart = BodyParts [i - 1];

         dis = Vector3.Distance (PrevBodypart.position, curBodyPart.position);

         Vector3 newpos = PrevBodypart.position;

         newpos.y = BodyParts[0].position.y;

         float T = Time.deltaTime * dis / mindistance * curspeed; 

         if (T > 0.5f)
             T = 0.5f;
         curBodyPart.position = Vector3.Slerp (curBodyPart.position, newpos, T);
         curBodyPart.rotation = Quaternion.Slerp (curBodyPart.rotation, PrevBodypart.rotation, T);

     }

 }



 public void AddBodyPart()
 { 
     Transform newpart = (Instantiate (bodyprefab, BodyParts [BodyParts.Count - 1].position, BodyParts [BodyParts.Count - 1].rotation) as GameObject).transform;

     newpart.SetParent (transform);
     BodyParts.Add (newpart);

 }
     

}